Voltage controlled oscillators (VCOs) are a class of devices that generate a periodic clock signal responsive to an input voltage. VCOs are often used in phase locked loops (PLLs) to generate a clock signal that is dynamically phase and frequency compared to an input reference clock signal. Typically, the output of a charge pump generates the input voltage to a VCO.
In known designs, VCOs and charge pumps are connected via a common node that in turn is connected to a voltage supply through a resistor and a capacitor in series. The voltage supply is typically ground. The charge pump alternately sources and sinks current to this common node responsive to the PLL's phase and frequency comparison. The resistor and capacitor thereby convert this current sourcing and sinking to a voltage level with alternating and direct components.
VCOs are frequently fabricated on a silicon substrate and integrated with other devices such as data processors, embedded controllers, digital signal processors, etc. Unfortunately, semiconductor processes are not able to fabricate resistors with a high degree of precision (value of resistance) or accuracy (repeatability of resistance over several devices). Therefore, systems incorporating known VCOs must be designed to operate with the range of resistances possible with a particular process.